What do you hope to get from NetBSD/i386 PAE in comparison to
NetBSD/amd64?

I have some older machines that will handle more than 4Gb memory but
from what I understand the CPUs will not run amd64 (missing EM64T
support; see below). Am I understanding the requirements for amd64
correctly?

If you can, boot a 5.0 install disk and look at its dmesg output -- I
was told a few months ago that 4.0 doesn't properly decode the feature
flags involved in the EM64T bit...